This invention relates to a sensor element for determining the O.sub.2 content of a sample, comprising an indicator substance fluorescing with the amount of O.sub.2 acting upon it, which may be brought into contact with the sample to be measured at least partially and which will give off fluorescent light upon excitation, the indicator substance being incorporated in a polymer carrier matrix in an at least approximately homogeneous distribution, and to a method of preparing such a sensor element.